Phosphorous sorption study of selected South African soils

Abstract

Phosphorous isotherms for 21 selected soils from the Republic of South Africa were constructed and the Langmuir, Freundlich and Temkin sorption models were compared using regression analysis. Over a range of solution concentrations of 0.025 - 2.00 mg P I⁻¹, all three sorption equations were found to fit the data well (R² = 0.934 - 0.996). The values of the coefficients of the different equations were more accurately estimated by the Freundlich and Temkin models, whilst the Langmuir model was a slightly better indicator of adsorbed P. A multiple regression analysis between sorption coefficients and selected soil properties showed citrate bicarbonate dithionite-AI to be an important factor in P sorption. Other soil constituents important in P sorption included clay percentage, organic matter, citrate bicarbonate dithionite-Fe and Bray 2 P content. Although useful in identifying soil properties responsible for P sorption, the regression equations were too cumbersome and imprecise to be of any practical use in characterising P isotherms routinely. It was also apparent from the regression analysis that, except for the Langmuir maximum and Temkin P buffer power index, the other sorption coefficients require the inclusion of a measure of native P content to be true soil characteristics, when constructing isotherms. Limitations, which this constraint imposes on the routine use of isotherms for the purpose of determining sorption coefficients, are discussed.